It seems to me as if the conditional mood often evolves from a construction that expresses future events from a past perspective. Why is that?

For example, it is obvious that the English construction would + infinitive is the past tense of will + infinitive, and it’s still used that way: “I knew you would come.”

But it also expresses hypothetical situations that have nothing to do with the past: “I would play tennis (right now, not at some point in the past), if the weather were better.”

The same thing applies to the Romance languages, where the future tense evolved out of a present–tense form of the Latin word for to have and an infinitive (for example, Italian amerò, “I will love,” comes from Latin amare habeo, literally: “I have to love”), and the conditional mood comes from a past–tense form of to have and an infinitive (amerei, “I would love,” ← amare habui, “I had to love”).

Is it because the second conditional originally referred to an event that didn’t happen in the past and was then reinterpreted as a construction that talks about hypothetical situations? Or is it because the implication is, “I would have been about to do something (and would therefore be doing it right now) if x had been the case”?